1. Field of the Invention
The present invention relates to a waveguide and an optical cable module for optical data transmission.
2. Description of the Related Art
Optical communication networks enabling communication of large volumes of data at high speeds have been expanding in recent years. The installation of the optical communication network on consumer equipments is anticipated in the future. In particular, an optical data transmission cable (optical cable) that can be used for electrical input and output and data transmission between substrates in equipment uses a waveguide in which the core and the clad can be made of a flexible high polymer material.
The waveguide is formed by a core having a large refraction index and a clad arranged in contact with the periphery of the core and having a small refraction index, and is used to propagate the optical signal entered into the core while repeating total reflection at the boundary between the core and the clad. If the waveguide having flexibility is used as the optical cable, alignment with photoelectric conversion elements (light receiving and emitting elements) and optical coupling must be performed. The light receiving and emitting element converts the electrical signal to the optical signal and emits the optical signal, and receive the optical signal and converts the optical signal to the electrical signal. Normally, a configuration of forming an optical path conversion mirror at the end of the waveguide is often used in coupling the waveguide with the photoelectric conversion element.
The configuration of connecting the waveguide and the photoelectric element using the optical path conversion mirror is shown in FIGS. 12A and 12B.
A waveguide 100 shown in FIGS. 12A and 12B is configured by a core part 101, an upper clad layer 102, and a lower clad layer 103. That is, the waveguide 100 has a stacked configuration in which the core part 101 is sandwiched by the upper clad layer 102 and the lower clad layer 103. The signal light transmitted by the waveguide 100 advances through the core part 101 while being reflected at the boundary of the core part 101 and the upper clad layer 102 or the boundary of the core part 101 and the lower clad layer 103.
The optical path conversion mirror in the waveguide 100 is formed by diagonally cutting the ends of the waveguide 100 to have the ends as inclined faces. In the waveguide 100 equipped with such optical path conversion mirror, the photoelectric conversion elements, that is, the light emitting element 111 and the light receiving element 112 are arranged in a direction perpendicular to the stacking direction of the core part 101, the upper clad layer 102 and the lower clad layer 103. In FIG. 12A, the light emitting element 111 and the light receiving element 112 are arranged below the lower clad layer 103.
In the above configuration, the transmission side exit light (signal light) from the light emitting element 111 passes through the lower surface of the lower clad layer 103 and enters the waveguide 100, and is then reflected by the optical path conversion mirror to become the light that advances in the optical axis direction of the core part 101. Specifically, the signal light is reflected by the inclined face at the end of the core part 101. The signal light advanced through the core part 101 is again reflected by the optical path conversion mirror at the reception side end of the waveguide 100, and then passed through the lower surface of the lower clad layer 103 to become a reception side incident light that enters the light receiving element 112.
In the configuration shown in FIGS. 12A and 12B, the reflection at the optical path conversion mirror must occur at the ends of the core part 101 in order for the signal light to advance through the core part 101. In reality, however, some light is reflected at the end of the upper clad layer 102 or the lower clad layer 103, as shown in FIG. 13, due to diffraction and scattering of light or due to alignment shift etc. at the time of mounting. Such light advances through the clad and is transmitted from the light emitting element 111 to the light receiving element 112. The light transmitted through the clad becomes a noise referred to as clad mode, which causes the lowering of the S/N ratio of the optical signal transmitted by the waveguide. Furthermore, in the waveguide equipped with the optical conversion mirror, under the condition that the mirror is angled at 45 degrees, the exit angle of the light from the light emitting element of ±15 degrees, the core thickness of 35 μm, the upper and lower clad thickness of 50 μm, and light emitting surface distance between the lower surface of the waveguide and the light emitting element is 300 μm, about forty percent of the light emitted from the light emitting element 111 becomes the clad mode.
A high polymer waveguide disclosed in Japanese Laid-Open Patent Publication No. 2004-199032 (Published on Jul. 15, 2004) has been proposed as a technique for preventing such clad mode. In the high polymer waveguide of Japanese Laid-Open Patent Publication No. 2004-199032, black organic pigment is added and dispersed in the upper clad layer and the lower clad layer. The light that has entered the clad layer is thus absorbed by the black organic pigment and the propagation of noise is inhibited.